1. Field
Some example embodiments may relate generally to silicon photomultiplier detector cells. Some example embodiments may relate to silicon photomultiplier detector cells that use digital signals.
2. Description of Related Art
Examples of techniques of imaging the inside of a human body include positron emission tomography (PET), magnetic resonance imaging (MRI), and X-ray computed tomography (CT). PET is a nuclear medicine imaging method in which an emitted positron is detected using radiopharmaceuticals to show physiological, pathological images of a human body. In PET, an analogue of glucose called F-18-FDG, which is a radioactive isotope, is injected into a body, and radiation produced due to a reaction between the analogue of glucose and a cancer existing in the body is detected after the lapse of a certain period of time (for example, several tens of minutes) to obtain information about the position of the cancer. A detector that detects a radiation produced due to a reaction between an analogue of glucose and a cancer in a body is referred to as a silicon photomultiplier detector or a gamma ray detector. Apparatuses for detecting radiation or gamma rays may be applied not only to radiation detectors for single photon emission CT (SPECT), CT, and the like, but also to various fields such as astronomy.
PET apparatuses are medical imaging apparatuses that inject radiopharmaceuticals for emitting positrons into a living body via an intravenous injection or intake, measure gamma rays produced due to positron annihilation by using a circular ring-shaped detector that surrounds the living body, and calculate the distribution of a positron-emitting nuclide within the body via a computer to thereby produce and show an image of the distribution. A photomultiplier tube (PMT) has been used as the detector.
PET has recently been incorporated with MRI, which provides anatomical information. In this case, a PMT causes distortion of an electrical signal under a strong magnetic field of MRI. Thus, there is a demand for a detector that is strong against the strong magnetic field of MRI, and a silicon photomultiplier (SiPM) detector has been proposed.
The SiPM detector may be roughly broken down into a scintillator, a pixel, and readout electronics. The scintillator transforms high-energy gamma rays of 511 keV into low-energy photons having a wavelength of 400 nm to 450 nm. Since a gamma ray is a very large energy photon, it is not absorbed by silicon but is mostly transmitted by the silicon. Thus, the scintillator transforms the gamma ray to have a wavelength band that can be absorbed by silicon. The pixel transforms an optical signal into an electrical signal by absorbing the photon obtained from the scintillator.
The pixel may be an analog type or a digital type. Each analog type pixel includes several thousands of microcells, each of which includes a single avalanche photodiode (APD) and a resistor. A signal of a microcell is turned on or off depending on whether a photon is incident upon the microcell. Thus, the intensity of a pixel signal is determined by summing all of the electrical signals generated by the microcells. The pixel signal is transformed into a digital signal for use in calculations of energy and time by the readout electronics. On the other hand, in a digital type pixel, some of application specific integrated circuits (ASICs) on a printed circuit board (PCB) of analog type pixels are implemented within a microcell. Thus, digital type pixels provide improved time resolution and improved energy resolution compared to analog type pixels. However, the time resolution and the energy resolution of SiPM still need to be further increased.